The overall goal of this proposal is to understand the genetic determinants of hyperoxia sensitivity in the cell. Hyperoxia, or exposure to higher than normal oxygen concentrations, is an important health problem since it can lead to serious lung-related injury. Presumably, toxicity from hyperoxia involves the production of reactive oxygen species; however, very little is known regarding the factors that influence this damage. Using Saccharomyces cerevisiae as a model system, genes that protect against hyperoxia damage will be identified and characterized. A genetic screen of the complete library of yeast deletion mutants has been initiated, identifying mutants that are sensitive to hyperoxia. Initial results indicate that protection against hyperoxia involves a set of antioxidant factors that is distinct for this type of oxidative stress. In future studies, this genetic screen will be completed (Aim 1), and we will begin to address the mechanisms underlying hyperoxia damage (Aim 2). The mutants will be tested for sensitivity to other oxidants and a potential role for metals in hyperoxia toxicity will be explored. One particular hyperoxia mutant has been selected for analysis in detail: pos5 (Aim 3). POS5 encodes an uncharacterized protein, that based on homology, may influence cell surface ferrireductase activity. The possible connection between ferrireductase, iron and POS5 in guarding against toxicity from hyperoxia will be explored.